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Fighting Corn Rootworm with Natural Predators Just Might Work

08/26/2016

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A handful of scientists believe better corn rootworm control is closer than we think — specifically, just under our feet.

Your average cornfield hosts an impressive array of life. Microbes, bacteria, fungi, nematodes and insect predators like spiders, mites and beetles can lurk in every square foot of soil. There’s growing evidence they could be deployed against corn rootworm larvae.

“We need to fight fire with fire,” said Jonathan Lundgren, an independent agroecologist and entomologist, who has been studying the natural predators of corn rootworm for more than a decade. “Corn rootworm is a very plastic and dynamic critter and we need to use something equally plastic and dynamic to fight it. Why not use what Mother Nature made a long time ago?”

While Lundgren has been studying how to encourage insect predator communities, Cornell University entomologist Elson Shields has spent the better part of his career successfully deploying local nematodes against the devastating alfalfa snout beetle in New York. He’s now found that a combination of two nematode species may decimate rootworm populations.

Success from these scientists would be a boon to corn growers. Western corn rootworm has evolved resistance to nearly every chemical and biotech tool deployed against it in the past few decades. Biological control options could supplement both Bt and the next generation of RNAi rootworm traits, and perhaps supplant some of them.

THE NEMATODE WHISPERER

Shield’s beetle-eating nematodes are the ideal farm investment. Alfalfa farmers in New York inoculated their soil with them once, using an evening field surface spray that cost $26 an acre. Now more than a decade later, those nematodes are still completely suppressing alfalfa snout beetle populations in alfalfa fields.

Their taste for rootworm larvae was discovered when Shields turned his attention to how well the nematodes survived if farmers rotated out of alfalfa to another crop, such as corn. To his surprise, the nematodes not only survived the corn rotation, but their numbers increased. “Their increases appear to coincide with when we see rootworm move in,” Shields said.

Two years ago, he inoculated a continuous corn field and a corn-soybean rotation field with the nematodes and set up some untreated and Bt-corn fields nearby. The 2015 season proved too light a rootworm year to collect data. But in 2016, rootworms gnawed away at corn roots in the untreated control, causing up to 1.9 nodes of damage, Shields said.

Fields with his nematodes performed exactly as well as the Bt fields, which were planted to Yieldgard, Herculex, and Smartstax hybrids. “The key is that the nematodes were applied two years ago,” Shields marveled. “We’ve worked really hard at keeping those persistent characteristics in these populations.”

Much more than one year of data is needed to confirm the nematodes’ efficacy against rootworm, but the results are so promising others are jumping on board. Monsanto has funded a project with USDA scientists in Columbia, Missouri, to find strains of nematodes in the Midwest that target the rootworm.

Some of these soil nematodes are attracted to rootworm-damaged corn roots, so the goal of the funded proposal is to help control Bt-resistant rootworm populations by targeting damaged Bt-corn roots with them.

“We need two things,” said USDA-ARS entomologist Bruce Hibbard, an advisor on the Missouri project. “We need strains that overwinter here in the Midwest, and we need to figure out how to maintain them for as long as possible. That might require some alternate sources of food, such as cover crops.”
COVER CROPS: FODDER FOR ROOTWORM PREDATORS?

Lundgren has long promoted winter cover crops to suppress rootworm populations, though not specifically for nematodes. His research is focused on larger, more visible field warriors — spiders, ants, centipedes, beetles and other insects.

Lundgren’s work revealed that rootworm blood has a repellent quality that keeps many biting insects at bay. However, sucking insects like spiders and ants appear to feast on the rootworm quite happily.

Cover crops can lure these predators to your field, Lundgren says. His work in South Dakota showed that cornfields planted after a winter cover crop of slender wheatgrass had higher insect predator populations and less rootworm damage than fields that lay bare over the winter.

Researchers found populations of ants, beetles and other insects, many of them with bits of rootworm DNA in their tiny tummies. “Overall, we’ve identified dozens of predator species as being important consumers of corn rootworm,” Lundgren said.

HUNTING FOR FUNGAL WEAPONS

Like Shields, Peterson and Hofman have been looking to the soil for rootworm solutions. For two years, Hofman dug up hundreds of soil cores from five irrigated cornfields in southwest Nebraska. She used a common fishbait insect called the waxworm to lure the fungi. When they infected her bait, she collected the fungi, grew them out in petri dishes and identified them. Now, armed with a library of local Nebraska fungal insect diseases, Hofman will see which ones attack rootworm larvae. The project is funded by the Nebraska Corn Board.

Peterson and Hofman also will test any promising fungal candidates against non-target insects to protect beneficial insect populations.

The biological organisms these researchers are working on may be rootworm solutions in and of themselves, but they are most likely to be supplements to the system in place, Peterson said.

“The great thing about biological options is they can be completely compatible with Bt traits,” she said. “We can get less broad-spectrum insecticide use, which will help conserve these natural enemies.”

Lundgren has even more ambitious hopes for biological rootworm control. He wants growers to stop seeing corn rootworm as a target for insecticides and other controls, but rather a warning sign.

“Corn rootworm isn’t the problem, it’s a symptom telling you something in the field is out of whack,” he said. He believes corn monocultures planted year after year into tilled fields have banished the valuable inhabitants of fields and soils, and with them, the rootworm’s natural enemies.

“We are creating our own rootworm problems by reducing biodiversity in our cornfields,” he said. “When you have a diverse insect community, then rootworms aren’t an issue anymore.”

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